In general, a multi-plate clutch in which a plurality of friction plates comprising a metal substrate (a core plate) with a wet friction material adhered to the surface thereof, and separator plates comprising a single plate such as a metal plate or the like as a friction mating material are alternatively positioned is integrated in automotive automatic transmissions, and the driving force is transmitted or interrupted by pressure contact or release of these plates to each other in the ATF used as a lubricant.
Rubber-based friction materials and so-called paper friction materials comprising paper as a base are used as wet friction materials. Paper friction materials are generally produced by mixing various friction modifiers or the like with pulp, then performing wet papermaking, and next, impregnating the resulting material with a binding resin such as a phenol resin, followed by curing. Paper friction materials have a high dynamic friction coefficient.
The automotive industry has recently been advancing weight reduction and high efficiency in various parts in the pursuit of energy efficiency and reduced weight. On the other hand, there is a tread for higher revving and higher power output of automobile engines. In automatic transmissions as well, friction materials having improved friction coefficients, heat resistance, and durability are desired in order to respond to the higher revving and higher output of automobile engines.
Namely, wet friction materials with a high heat resistance at high temperatures and high-load conditions have been sought, and there is also a strong demand for even greater improvements in high friction coefficients.
In order to improve on these problems, trials to increase the strength of a material using a nonwoven fabric were made for a wet friction material described in Patent Document 1, for example. However, this has drawbacks in that the base material per se is expensive, and in that the friction coefficient thereof is gradually reduced during use because only the friction modifier per se adheres to the surface thereof, and the adhesion strength itself is not very strong. In addition, Patent Document 2 proposed a process comprising a step of impregnation with a thermosetting resin in which fine particles of a mesophase pitch are mixed and dispersed, followed by a step of processing at a high temperature to carbonize the mesophase pitch and the thermosetting resin to prepare the base paper material, whereby high strength and porosity can be maintained. However, friction materials prepared by these processes have a remarkably reduced friction coefficient, compared with general paper friction materials.
Carbon ingredients are useful substances to control the frictional force, and may be used in various forms.
Patent Document 3 describes a friction material using carbon nanofibers. It also describes the use thereof for clutch facings and the like as well as disc brakes for cars. Further, it describes improved rotational fracture strength as the effect of the friction material. Therefore, the technical content of Patent Document 3 is different from that of the present invention which is aim at improving the high friction coefficient and heat resistance (heat spotting resistance) of a friction material used in an oil.    Patent Document 1: Japanese Patent Laid-open No. 2004-217790    Patent Document 2: Japanese Patent Laid-open No. 11-5850    Patent Document 3: Japanese Patent Laid-open No. 2004-217828